Electrode assembly for detecting particles in fluid suspension



Dec. 8,1970.

ELECTRODE ASSEMBLY FOR DETECTING PARTICLES IN FLUID SUSPENSION FiledSept. 9, 1968 J. D. PERRINGS /Z ACOUSTIC AcousIrIc DRIvER DRIvERGENERATDR Va 3 4 9 GAs PRESSURE /0 PRESSURE REDUCING MEANS MEANS [/2COLLAR PROPORTIONAL PULSE DELAY PULSE GENERAFR GENERATOR SENSOR 4 4?- r0PULSE r0 PULSE GENERATOR v GENERATOR Fig. 2

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James 0. Parr/figs BY United States Patent 3,546,583 ELECTRODE ASSEMBLYFOR DETECTING PARTICLES IN FLUID SUSPENSION James D. Perrings, LosAlamos, N. Mex., assignor to the United States of America as representedby the United States Atomic Energy Commission Filed Sept. 9, 1968, Ser.No. 758,561 Int. Cl. G01n 27/00 US. Cl. 324--71 4 Claims ABSTRACT OF THEDISCLOSURE An improvement in an electrode assembly for detectingparticles. Such an electrode assembly is used in a particle counter or aparticle separator for sorting minute particles present in a suspensionfluid in accordance with a selected characteristic such as size,conductivity or any other characteristic that can be translated into ananalogous electrical quantity. An improved form of the electrodes in thesensor prevents spurious modulations of the current due to gas bubbleaccumulation at the electrodes from interferring with the sensor signal.

The invention described herein was made in the course of, or under, acontract with the US. Atomic Energy Commission.

Apparatus for sorting particles suspended in a liquid in accord withsize or other desired characteristic is shown and described in US. Pat.No. 3,380,584, issued Apr. 30, 1968, to Mack I. Fulwyler. The sensingand sorting of particles suspended in a liquid is accomplished bypassing the liquid at high speed through a sensing nozzle, followed bythe fluid passing through a charging collar, then between two deflectingplates and finally into receptacles in accordance with the deflectionimposed at the deflection plates. The fluid is formed into droplets atthe sensing nOZZle and the sensor detects the information concerning thecharacteristics of each droplet that will later be used for sorting. Asthe droplet passes the charging collar an electric charge is imposed oneach droplet in accordance with the information detected at the sensingnozzle. Then as the droplets pass the deflection plates each dropletwill be deflected an amount dependent upon its particular charge andbecause the charge depends upon the information obtained at the sensingnozzle, the deflection is in accordance with the desired characteristic.

The present invention also relates to a device for counting particlessuspended in a fluid as shown by Coulter Pat. 2,656,508 wherein thesensing nozzle is a short dielectric duct or aperture of appropriatecross section through which the suspension fluid is forced to flow and apair of electrodes are disposed on opposite ends of the aperture incontact with the fluid. The electrical resistance between the electrodesis affected by the presence and size of a particle in the fluid withinthe duct.

Detectors, counters, and sorters of this type are useful in medicalresearch. Particles in body fluids such as blood cells may be counted oractually sorted in accordance with size or other characteristics. Theparticles to be counted or sorted are usually suspended in a liquid suchas a saline solution.

Early use of this type of device indicated that noise or static wasmasking the true signal. Investigation of the source of static led tothe discovery that the liquid was being dissociated into atomic gasparticles by the potential across the dielectric disc from the outerelectrode to the nozzle element. The impression of an electric chargeacross an aqueous solution results in the decomposition of water intohydrogen and oxygen gases which migrate to and are collected on theelectrodes. Hydrogen is a cation being ice positively charged andmigrates along electropotential lines to deposit on the cathode. Theoxygen, conversely, is an anion and collects as atoms of oxygen formingminute bubbles at the anode. The atomic gas rapidly accumulates on theexposed surface of the electrode as a growing bubble causing acorresponding increased resistance, and upon sufficient accumulation iswashed through the aperture, thus generating a current pulse similar tothat caused by a real particle.

Accordingly, a primary objective of the present invention is to providean electrode assembly which is not susceptible to the generation andrelease of gas bubbles in a manner which gives rise to electricalsignals confusingly similar to the signals generated by particles ofinterest.

Other objectives and advantages of the present invention will becomeapparent as the following description is read with reference to thedrawings.

Referring to the drawings,

FIG. 1 is a schematic diagram of a particle separator incorporating theelectrode assembly of this invention.

FIG. 2 is an exploded cross-sectional view of the electrode assemblyused in the prior art.

FIG. 3 is an exploded cross-sectional view of an improved electrodeassembly.

A particle separator that includes the sensing element of the presentinvention is shown in FIG. 1. Liquid containing particles in suspensionis stored in any suitable container 5. Means for pressurizing the fluidmay be any suitable means such as a pump in the outlet of the container5 or gas pressure means 3 and controllable pressure reducing means 4.

The fluid container 5 is connected by a strainer 6 and pipe 7 to nozzle10. Strainer 6 is provided to pass particles within the range ofinterest and to stop particles of gross size from clogging the system.Although the fluid in passing through the nozzle may be jetted andseparated into droplets by suitably vibrating the nozzle, it ispreferred to apply pulsations directly to the fluid and avoid vibrationof the nozzle. This method of drop formation is accomplished byresilient acoustic insulation 9 between nozzle 10 and acoustic coupler8. The acoustic coupler 8 is directly driven by an electrically drivenvibrator 9 such as a piezoelectric driver 2 which in turn is energizedby adjustable frequency generator 1.

The frequency and amplitude of the output of generator 1 is correlatedwith the viscosity and velocity of fluid passing through the nozzle inorder to produce equal size discreet droplets from the resulting jet.

The sensor element 14 is shown associated with the fluid containingsystem at the outlet of the nozzle. At the outlet of the nozzle thefluid has obtained its jet cross section and velocity so that thetransport time between the passage of the fluid through the sensor andthe point along the path of breakup of the jet into droplets is a fixedand calculable quality.

The sensor 14 is selected to be responsive to the particularcharacteristic of the particles of interest, i.e., it Will be responsiveto particle size, electrical conductivity, and any other characteristicwhich is capable of transformation into an electrical quantity. Theelectric pulse generated in the sensor by the passage of a particle isamplified and shaped by proportional pulse generator 13. The utilizationof the pulse to detect the associated droplet is delayed an amount oftime equal to the time consumed by the fluid containing the particle intraversing the distance from the sensor to the point where the liquid isdetaching from the jet as a droplet. The pulse, after appropriate delayin delay element 12, controls the amplitude and time of generation ofdroplet charging potentials. More specifically, each droplet or group ofdroplets is charged by collar pulse generator 11 in response to thedelayed pulse from delay element 12. The collar pulse generator 11produces the proper potential which is inductively impressed along thejet stream between charging collar 15 and sensor 14. The charging collarsurrounds the jet stream at a location which includes the dropletseparation zone. Thus, the potential which exists between the collar andthe sensor at the time of Separa tion of the droplet determines thebound charge on the droplet. The droplet or train of droplets continuesin the direction of the jet stream until it passes between defleetingplates 16 and 17. Deflecting plates 16 and 17 are energized with asteady-state potential of suitable magnitude. Each droplet in passingbetween the deflection plates is deflected by an amount determined bythe charge on the droplet. The droplets are thus segregated inaccordance with the amount of a selected characteristic and can becollected in any desirable manner.

A sensing nozzle of the prior art is shown in FIG. 2. The nozzle element10 constitutes one electrode of a suitable noncorrosive metal such asstainless steel, platinum or gold. A dielectric disc 19 having anaperture 20 is affixed in any convenient manner in liquid leak-prooffashion to the end of the nozzle element 10. A second electrode 21having an aperture 22, the walls of which contact the liquid, is afiixedto the downstream face of the dielectric disc 19 also in liquidleak-proof fashion. Liquid is forced into the nozzle by inlet tube 7 andis vibrated in the manner described above. The nozzle element 10 and theend electrode 21 are impressed with a difference in potential to detectchanges in electrical conductivity as particles pass through thedielectric disc 19. As explained above, the electric charge across thesolution causes bubbles to be formed and accumulated at the elec trodes19 and 21. Periodically the bubble accumulation is washed through theapertures and a false signal generated that masks the true signal ofparticles passing through the apertures.

It is not considered possible to prevent the formation of gas bubbles,but by causing the gas bubbles to be formed so that the electrical pulsethey generate is of a range outside the range of the particles ofinterest it is possible to eliminate the masking effect. Hence, thesignal-to-noise ratio is greatly improved and not only is the accuracyof the device improved but the range of particle size to be investigatedis increased.

One embodiment of the improved nozzle assembly 14 incorporatingapplicants invention is shown in FIG. 3. The nozzle element constitutesone electrode of a suitable noncorrosive metal such as stainless steel,platinum or gold. A dielectric disc 19 having an aperture and recessedcavities 23 and 24 is affixed in any convenient manner in liquidleak-proof fashion to the end of the nozzle element 10. A secondelectrode 21 having an aperture 22 and recessed cavity 25 is affixed tothe downstream face of the dielectric disc 19 also in liquid leak-prooffashion. As in the prior art, liquid is forced into the nozzle by inlettube 7 and is vibrated. The nozzle element 10 and the end electrode 21are impressed with a difference in potential to detect changes inelectrical conductivity as particles pass through the dielectric disc19. The gas bubble accumulation caused by the charge across the solutionfills the recessed cavities before being periodically washed through theapertures. The bubble accumulation is of a greater magnitude than priorart devices because a greater portion of the electrodes 10 and 21 areexposed to the liquid and the recessed cavities 22, 24 and 25 allow moreroom for accumulation. Consequently, when the bubble accumulation isperiodically washed through the device the pulse generated will be largeand outside the range of the particles being investigated. In addition,the periodic washing through of bubbles is less frequent. This allows aneasy separation of the false signals from those generated by trueparticles. Various forms and shapes of the recessed cavities arecontemplated dependent upon the size of the particles beinginvestigated, the accuracy desired and the instrumentation to be used.

What I claim is:

1. In an apparatus for sensing small particles suspended in a fiuidmedium having sensor means for generating an analog electrical signal inresponse to the passage of a particle, the improvement comprising anelectrode assembly sensor which eliminates spurious signals caused bygas bubbles formed on the electrodes, said assembly having first andsecond aligned electrodes each having a central aligned apertureseparated by a dielectric spacer element also having a central aperture,the dielectric element having a countersunk seat on each end portion oflarger diameter than the aperture to accommodate the accumulation of gasbubbles which are periodically washed through the device, the bubbles soaccumulated being distinctly larger than the small particles insuspension so that the sensor generated signal triggered by the passageof the bubbles is identifiably different from that generated by thepassage of a particle.

2. The improved electrode assembly of claim 1 wherein the secondelectrode has a centrally disposed countersunk seat located on an endportion of said electrode surrounding the aperture and facing the spacerelement.

3. The improved electrode assembly of claim 2 where the countersunkseats are disc shaped.

4. The improved electrode assembly of claim 2 wherein the countersunkseats are hemispherical in shape.

References Cited UNITED STATES PATENTS EDWARD E. KUBASIEWICZ, PrimaryExaminer

